Loud-speaking telephone system



Feb. 15, 1944. M G|ANN|N1 2,341,539

LOUDSPEAKINQ TELEPHONE SYSTEM Filed Nov. 14, 1941 5 Sheets-Sheet 1 I04100 Signal (lament flmlz'fz'er Confro/ Bal Hybrid .Q I Nei 10p Neflllk35.51%? 15 INVENTOR Gabriel M. Giannini BY ATTORNEYS Feb. 15, 1944.

G, M. GIANNINI 2,341,539

LOUDSPEAKING TELEPHONE SYSTEM Filed Nov. 14, 1941. 5 Sheets-Sheet 2*QQQSLQQQQQQat- F INVENTOR Gabriel M. Giannini ATTORNEYS Feb. 15, 1944.G; M. GIANNINI I LOUDSPEAKING TELEPHONE SYSTEM 5'Sheets-Sheet 5 FiledNov. 14, 1941 3039'; Balancing NeZLw/"k 3Q4 300 i ybrzcoz Syszem 309 F 3v INVENTOR.

. Gabriel M Giannini I. I BY I If I l g E 2 flztorneys Feb. 15-, 1944.

s. M. GIANNINI LOUDSPEAKING TELEPHONE SYSTEM Filed Nov. 14. 1941 5Sheets-Sheet 4 e 1 m 4 efi ..1. WWW

G. M. GlANNlNl 2,341,539

5 Sheets-Sheet 5 Feb. 15, 1944;

LOUDSPEAKING TELEPHONE SYSTEM Filed Nov. 14, 1941 I Patented Feb. 15,1944 v LOUD-SPEAKING TELEPHONE SYSTEM Gabriel M. Giannini, NorthHollywood, calm, as-

signor, by mesne assignments, to Automatic. Electric Laboratories, Inc.,Chicago, 111., a corporation of Delaware Application November 14, 1941,Serial No. 419,165

' 17 Claims. ('Ci. 179-=-l) The present invention relates toloudspeaking telephone systems and, more particularly, to improvementsin methods and apparatus for increasing the stability of operation ofsystems of this character.

Aside from the so-called manually controlled push-to-talk andpush-to-listen" arrangements, loud-speaking intercomrnunicating systemsof the character mentioned usually. take either of two forms. In oneform of the system, as exemplified by Patent No. 2,208,160, Harold J.McCreary, granted July 16, 1940, separate transmitting and receivingchannels, each provided with signal current amplifiers, are utilized inthe transmission of signal currents between two stations arranged forloudspeaking service. In the other type of system, as exemplified byPatent No. 2,224,477, Roswell H. Herrick, granted December 10,1940, asingle two conductor channel is utilized for transmission in eitherdirection, each station connected to the channel usually being providedwith transmitting and receiving amplifiers and a hybrid system forobtaining side tone suppression in the station circuit. In either typeof system, the problem of preventing the acoustical coupling between thetransmitting and receiving elements from setting up an oscillatorycondition in the system is difllcult to obviate. Thus. in the twochannel arrangement, the acoustical coupling at the two ends of thechannels provides a closed signal transmission circuit which includesthe two signal amplifiers. This closed circuit tends to oscillate whenthe gain through the amplifiers exceeds the acoustical and electricallosses of the circuit. Again, in the two-conductor single channelarrangement, each substation network eflectively comprises a closedcircult which includes the transmitting and receiving amplifiers, theacoustical coupling between the microphone and loudspeaker, and theelectrical coupling through the hybrid system. This closed circuit willlikewise oscillate when the gain through the amplifiers exceeds theelectrical and acousticaliosses of the closed station circuit. Theabove-noted common characteristic of the two systems places a definitelimitation on the" amount of gain which maybe introduced into the signaltransmission channels before singin level.

- extent possible in order to raise the permissible erating both thetransmitting and receiving channels at a sub-normal gain setting and forinversely varying the gains of thesignal amplifiers in response tosignal current transmission, so that the gain of the channel in useisraised to a normal value and the gain of the inactive channel isdecreased .by at least a corresponding amount. While this and likearrangements may be designed satisfactorily to eliminate singing, itgives rise to ancillary problems. For example, with the gain of theinactive channel at an extreme sub-normal value, conversation break ins"can only occur at a very low transmission Also, the problem ofpreventing speech clipping" is diflicult to obviate.

It is an object of the present invention, therefore, to provide animproved system of the character described wherein the transmitting andreceiving elements provided at each station of the system areacoustically decoupled to the greatest level of signal current gainthrough the transmission channel in use, and wherein the system is soarranged that stability is maintained with an attendant satisfactorygain in the active channel and without an objectionble decrease in thegain of the inactive channel.

or howling will occur, and hence places a'limitation on the volume atwhich received speech cur-' rents may be transmitted and reproduced.Various so-called voice operated switching arrangements have beendevised foreliminating this difliculty. In at least one system of thisgeneral character, facilities are provided for normally op-' atedelements, whichspacing isdetermined by the point in the combinedfrequency response characteristic of the two elements at which maxi-'mum coupling between the elements tends to occur. 7

According to a further object of the invention, the loudspeaker andmicrophone elements of the system are of improved construction,characterized by an absolute'minimum of mechanical or conductivecoupling between the associated ele ments at each station, and utilizediaphragms which are so proportioned as to size that the acousticalcoupling between associated elements is further minimized.

According to a still further object of the invention, sound transmissionfrom the loudspeaker or receiving element at each station is eflectedalong a substantially vertical path, while sound input to. theassociated microphone is eilected along a substantially horizontal path,thercby further to enhance the acoustical impedance between the twoelements.

According to still another object of the invention, directional soundwave transmission from the loudspeaker at each station is utilized as avehicle to minimize sound wave reflection and refraction into theresponse zone of the associated microphone during operation of theloudspeaker, thereby further acoustically to decouple the two elements.

It is another object of the invention to provide an improved method ofoperating the system, whereby the two transmission channels are normallyoperated at the maximum gain consistent with stability of the system,and the inverse changes in the gains of the two channels which occurduring signal current transmission are limited to a narrow range justsufficient to preserve the stability of the system.

In accordance with another object of the invention, the operation of thegain control apparatus is prevented until the amplitude of the sig-' nalinput to the active channel attains a predetermined value which exceedthe normal noise current level.

According to still another object of the invention, the gain controlapparatus for the signal amplifiers of the two channels is provided witha frequency discriminating network which favors the frequencies in thelower portion of the operating frequency range, within which the majorportion of the sound energy developed by the human voice is generated.

The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the specification taken in connection withthe accompanying drawings in which Figs. 1, 2, and 3 illustrate thecircuit arrangement of an improved system having incorporated thereinthe features of the invention briefly referred to above; Fig. 4aillustrates one improved loudspeaker-microphone arrangement which maybeused at each of the stations of the system; Figs. 4b 4c and 4dillustrate a modified. arrangement of the loudspeaker-microphone unit;Fig. 4e illustrates another modification of the loudspeaker-microphoneunit; and Figs. 5 to 9, inclusive, illustrate the details of thetransmitting and receiving elements provided at each of the substationsof the system.

Referring now more particularly to Figs. 1, 2, 3 and 4 of the drawings,the improved loudspeaking intercommunicating system there illustratedcomprises, in general, an east station l0 and a west station II whichare interconnected by a pair of lines I 3 and I4 having a centralrepeater l2 connected therebetween. This repeater may be provided at acentral point in the system and may be used to stabilize thetransmission level over the two connected lines l3 and I4. Brieflydescribed, this repeater comprises an east-west amplifier a west-eastamplifier 200, a pair of hybrid systems 202 and 203, balancingnetworks204 and 205 individual to the hybrid coils and respectivelylines l3 and I4, and a signal operated control netthe two amplifiers 200and 2M in accordance with the direction of signal current transmissionover the lines l3 and I4.

More specifically considered, the west-east amplifier 200 comprises acoupling transformer! having its input winding connected to thesignaltransmitting terminals of the hybrid system 203,

and its output or secondary winding coupled to the input electrodes201!) and 2010 of the threetrodes 201a and 2010 of this tube are coupledto the signal input terminals of the hybrid system 202 through acoupling transformer 208. The opposite or east-west amplifier 20lsimilarly comprises a coupling transformer 2 having its primary windingconnected to the signal transmitvting terminals of the hybrid system202, and its secondary winding coupled to the input electrodes 2i2b and2l2c of the three-electrode amplifier tube M2. The output electrodesH211 and 2 We of this tube are coupled to the signal input terminals ofthe hybrid system 203 through a coupling transformer 2 l3. Preferably,the amplifier tubes 201 and H2 are of the well-known 605 type, utilizingindirectly heated cathodes and characterized by a comparatively highamplification factor.

The control network 220, as indicated above, is provided for the purposeof increasing the gain of the sign-a1 amplifier 200 or 20! which is inuse during signal current transmission over the two lines l3 and I4, andfor concurrently effecting a corresponding decrease in the gain throughthe signal amplifier included in the inactive channel. Briefiydescribed, this network comprises a three-electrode amplifier tube 223having its input electrodes 223b'and'223c coupled to the input circuitof the west-east channel amplifier tube 201 through an a'dustableVoltage dividing resistor 222 and a coupling transformer 22!. The outputelectrodes of the tube 223 are coupled to one rectifying section 23l a,23Ic of a duplex diode 23l, through a coupling transformer 224. Theother section 23ld, 23le of the vacuum tube 23! is coupled to the inputcircuit of the east-west channel amplifier tube 20l over a signalcurrent path which includes the coupling transformer 228, thethree-electrode amplifier tube 221, the adjustable voltage dividingresistor 226 and the coupling transformer 225. The duplex diodie 23lfunctions to rectify signal currents appearing across the secondarywindings of the coupling transformers 224 and 228, re-

rent amplifier tubes 232 and 233 in opposite senses. Thus, it will benoted that the diode section 2311:, 23lc is included in a closed directcurrent circuit which includes the space current path between theseelectrodes, a resistor 230, the

secondary winding of the transformer 224, and a resistor 234. The othertwo electrodes 23ld, 23Ie of the duplex diode 23l are similarly includedin a circuit which includes the resistor 234 the secondary winding ofthe transformer 228 and a resistor 229. A smoothing condenser 236 isconnected in shunt with the resistor 234 in order to smooth outalternating components of the voltage developed across this resistor andto determine the time constant of the control circuit. The voltagesdeveloped across the two halves 234a and 234b of the resistor 234 areimpressed in opposite senses across the'input electrodes of twothree-electrode control tubes 232 and 233:1!- spectively, through theadjustable cathode biaswork 220 which is operative to control the gainof ing resistors 235a. and 235b, respectively. The

output circuit .of the tube 232 includes a load resistor 236a which isshunted by a smoothing condenser 231 and is so connected that thevoltage developed thereacross is impressed as a bias voltage between theinput electrodes of the westeast amplifier tube 201. More particularly,the

- resistor 236a is connected to the control grid 201b'of the tube201over a path which includes the secondary windingof the transformer 206.

electrode amplifier tube 201. The. output elec- 16 This winding isbridged by a-terminating network which comprises the shunt-connectedresistor 209 and condenser 2I0. Similarly, the output circuit of theother tube '233'includes a biasing resistor 23Gb which is shunted byasmoothing condenser 230 and is connected through the secondary windingof the coupling transformer H I to the control grid 2I2b of theeast-west amplifier tube 2I2. This secondary winding is also bridged bya terminating network which includes theshunt-connected resistor 2I3 andcondenser 2 I4. Preferably, the'control and amplifier tubes 223, 221,232 and 233 are of the commercial type 605, and the duplex diode is ofthe 6H6 type.

Anode voltages are impressed upon anodes 201a and 2I2a of the two signalamplifier tubes 201 a control network 3I6 is'provided. The various anodeand biasing voltages required for operation of the amplifier tubes andthe tubes of the control network 3I5 are derived from a. power packindicated generally at 335.

More specifically considered, the transmitting I amplifier 300 comprisesa thermionic tube 306 of and 2I2 from the voltage source 2, over pathswhich respectively include the signal current choke coils 245 and 2I5.source 24I are by-passed for signal currents by the condensers 244 and2I6. In this regard it will be noted that the cathodes M and 2010 of thetwo amplifier tubes are connected to the tapped point along the voltagedividing resistor 242 in order to provide the required potential onthese cathodes. The voltage of the source MI is also positively appliedto the anodes of the These coils and the the well-known pentode type,having its input electrodes 306b'and 306a coupled to one of the threemicrophones 4M, 4 and 42I, over a chan- -nel which includes anadjustable voltage dividing resistor 305, a condenser 363, a couplingtransformer 304 and the cord conductors 430. The output electrodes ofthis tube are directly coupled to the input winding of the hybrid system302, which is shunted by a resistor 366. The other signal amplifier Isimilarly comprises a thermionic tube 308 of the well-known pentodetype, having its input electrodes 30811 and 3080 coupled to thereceiving terminals of the hybrid system 302 through an adjustablevoltage dividing resistor 30?.

420 in use,'over a channel which includes the coupling transformer 309and the cord conductors 43I. The two tubes 306 and 308 are provided withcathodes 3000 and 3080 which are of the filamentary type, such thatrapid changes in the divider 239, which divider is bridged across the.

section 24%. of the resistor 242.

Referring now more particularly to the equipment provided at the eaststation l0; this equipment is illustrated in Fig. l of the drawings ascomprisin a transmitting element in'the form of a microphone I00, areceiving element in the form of a loudspeaker'IOI, a hybrid system I02,and a balancing network I03 which is provided to balance the impedanceof the line I3 so that signal energy transfer between the transmittingand receiving channels of the station through the hybrid system isreduced to a minimum. The station equipment further comprises atransmitting amplifier I04, a receiving amplifier I06, and a controlnetwork I06, which functions to control the signal 'current gain through;the' two amplifiers in the manner more fully pointed out hereinafter.

In general, the arrangement of the equipment provided at the westst'ation II is identical with that provided at the east station I0 and,accordingly', only the details of the station II have been illustratedin the drawings. Briefly described, the equipment provided at the weststation I.I

includes one of the three loudspeaker-microphone units illustrated inFig. 4 of the drawings, and

' the circuit equipment illustrated in Fig. 3 of the electron emissionof-the cathodes is obtained in response to changes in the currenttraversing the cathodes.

The input and output terminals of the control network 3I5 are coupled tothe receiving channel of the station circuit at a point just followingthe signal amplifier tube 308. Briefly described,

this network comprises a signal amplifier tube' 322 having its inputelectrodes 322D and 322c transformer 324 to therelectrodes of. a triode326 which is connected to o erate as a diode rectifier.

A load resistor 328 sh ted by a signal current by-pass condenser 329 isconnected in circuit with the space current path of the tube 326, and

the voltage'developed thereacross is utilized to control the potentialdifference between the input electrodes 33% and 330001? the control tube330. In order to'prevent the control circuit 3|! from effecting a changein the gain of either of the two channel amplifier tubes 306 and 300when the signal input to the station II is below a predetermined value,a biasing battery 325 is providedin circuit with the tube 326.

functions to prevent the tube 326' from passing, current until thesignal input to the station exceeds the selected predetermined value.

transmitting and receiving terminals of a hybrid system 302. 'Abalancing network 303-01 the adjustable resistive type is provided forbalancing the impedance of the line I4 to minimize energy.

transfer between th transmitting and receiving channels of the-stationII. controlling the gain of the two amplifiers 300 and 30I to preservestability of the station circuit,

The control tube 330 is provided-with a space current path between itsoutput electrodes 330m and'330c which may be connected in parallel withthe space current path of the signal amplifier tube 308, and in serieswith a cathode biasing resistor 352. It will be apparent that with theFor the purpose of space current path of the tube 330shunting the outputcircuit of the amplifier tube 308, the signal current gain through thereceiving amplifier 30I This battery is varied directly in accordancewith the resistance of the space current path through the tube 330. Itwill also be noted that the cathode biasing resistor 352 is shunted bythe cathode 3060 of the transmitting amplifier tube 306, whereby theelectron emission of the tube 306 is controlled in accordance with thecurrent traversing the space current path of the tube 330. This cathodeis shunted by a large condenser 365 which functions to by-pass thecathode for signal frequency currents.

' 349 through a filter network which comprises a series choke 346, apair of shunt connected filter condensers 347 and 348, and a bleederresistor 350. The mid-point of the high voltage winding 339 is connectedto the low voltage terminal of the load resistor 349 and to groundthrough a load resistor 343 which is tapped at two points to providebias voltagesfor the two tubes 306 and 330. Thus, the voltage ofapproximately 5.5 volts which is developed across the section 343aof-the resistor 343, during operation of the apparatus,

7 is negatively applied to the control electrode 30% of the tube 306over a path which includes the filter resistor 364 and the lowersectionof the voltage dividing or volume control resistor 305. Thissection of the resistor 343 is shunted by an alternating current by-passcondenser 344 to prevent noise from being introduced into the signalamplifier'300 from the power pack. The voltageof approximately 18.5volts which is developed across the section 34311 of the resistor 343during operation of the power pack 335, is negatively applied to thecontrol electrode 33011 of the tube 330 over a path which includes theresistor 328. This section of the resistor 343 and the resistor 328 areby-passed for alternating currents by the condensers 33l and 345. Theentire voltage of approximately 2'70 volts developed across the loadresistor 349, during operation of the power'pack 335, is positivelyapplied to the anode 308a of the receiving amplifier tube 308 over apath which includes the resistor 362 and the primary winding of thecoupling transformer 389. This, voltage is also positively applied tothe screen electrode 308d of the tube 308 over a path including theresistor 362. Depending upon the position of an on-off switch 36I,-which is provided for rendering the control network 3l5 active orinactive as desired, this'voltage is also either directly applied to theanode 330a of the tube 330 or is applied to this anode through theresistor 362 and the primary winding of the transformer 309. A portionof the voltage developed across the load resistor 349, i. e.,approximately 110 volts, is positively applied to the anode of thesignal amplifier tube 322 through the primary winding of the couplingtransformer 324, and to the anode of the transmitting amplifier tube 306over a path which includes the resistor 366 and the signal input windingof the hybrid system 302 in parallel. This voltage is also directlyapplied to the screen electrode 306d of the transmitting amplifier tube306.

A by-pass condenser 321 is connected in shunt with the portion of theresistor 349 from which 'the cathode 3030 is normally maintained at apotential which is substantially positive with respect to its associatedcontrol grid This biasing path is by-passed for alternating currents bya shunt connected condenser 353. Cathode heating currents are suppliedto the cathodes of the three tubes 322, 326 and 330 from the three lowvoltage windings 351, 356 and 358 of a voltage step-down transformer354, this transformer be- 7 ing provided with a primary winding 355which acoustical damping material, such, for example, as

shunted by a signal current by-pass condenser 360.

The third low voltage winding 358 is connected to supply cathode heatingcurrent to the cathode 3300 of the tube 330 and is provided with atapped center point. which is connected to ground'through the cathodebiasing resistor 352.

' As indicated above, any one of the threelcudspeaker-microphonearrangements illustrated in Figs. 4a, 4b, 4c, 4d and 4e of the drawingsmay be used in conjunction with the circuit equipment shown in Fig. 3 ofthe drawings. The common purpose of the three arrangements is tominimize acoustical coupling-between the loudspeaker and the microphoneand thus reduce the tendency of the station circuit to oscillate andproduce singing or howling. Briefly considered, the unit illustrated inFig. 4a of the drawings comprises a microphone 40! which is arranged tobe primarily responsive to sound waves I transmitted thereto along ahorizontal path, and a loudspeaker 402 which points upward and isarranged to transmit sound waves in a substantially vertical direction.These two translating devices are housed in a casing 400a which isprovided with curved outer surfaces, and in order to minimize conductiveor mechanical coupling between the two-translating elements 40l and 402,the casing 400a is filled with sets of elongated sound transmissionpassages 4 and M3 of tubular form, which respectively communicate withthe chambers adjacent the diaphragms of the two translating elements.-In this arrangement, also, the loudspeaker M2 and the microphone 4 arehoused in a casing 011 and r are surrounded by felt or other acousticalinsulating. material to minimize the conductive coupling therebetween.trated in Figs. db, 4c and 4d, the sound chamber i adjacent thediaphragm of the loudspeaker 122 communicates with: two sets of soundpassages lilo and izlb which extend to the air surrounding the unit.These passages, at their outer ends, are flared away from each other sothat the sound emitted therefrom is directed away from the zoneintermediate the two sets of sound transmission 1 openings. Thetransmitter element or microphone ii2i is disposed between the two setsof sound passages 424a and 424b and is provided with a diaphragm whichcommunicates with the air surrounding the unit through the elongatedsound passages 423. As'best shown in Fig. 4d of the drawings, thesepassages are directed slightly downward to prevent sound reflected fromthe walls and ceiling of the room in; which the unit is .located frombeing transmittedto the diaphragm of the microphone MI. The loudspeakerM2 and the microphone d2! are-housed within a alternative to thisarrangement, the two ele- U verge at the openings of these passages andare transmitted away from the zone in which sound is transmitted to thesound passages 423 which communicate with the diaphragm of themicrophone Mi. By virtue of this arrangement, a

minimum of the reproduced sound energy is fed back through the passages423 to the'air chamber adjacent the microphone diaphragm.

Further to consider the three'embodiments of the microphone-loudspeakerunits, it has been found that station transmitter receiv'ercombinations, in general, have a number of peaks in the combinedfrequency response characteristic @thereof, which peaks representparticular frequencies at which maximum coupling between the receiverand transmitter obtains. In other words,

' signal currents having frequencies corresponding to the responsecharacteristic peaksand reproduced by the receiver element, cause themaximum response of the associated transmitter ele- 'ment. It has alsobeen observed that the acoustical coupling between the stationtransmitter and receiver elements is affected by the spacing between thesound transmitting and sound receiving openings which connect themovable diaphragms of the coupled elements with the surrounding air.More specifically, the coupling is greatest when the spacing between thesound transmitting and sound receiving openings of the In thearransemcniiaillustwo elements is of the order of one-half of the wavelength representing the sound frequency under observation. Accordingly,when the openings of the sound transmitting and receiving elements arespaced apart by a distance equalto an odd number of quarter wave lengthsof a particularly objectionable frequency, a minimum amount of,acoustical coupling between the two elements and, hence, a maximum ofsystem stability, are obtained. Usually the most objectionable peak in.the combined frequency response characteristic of the transmitting andreceiving elements occurs in the lower portion of the voice frequencyrange. For example, tests on a-specific combination of elements haveindicated a maximum combined response of the two elements at a frequencyof-the order of 600 cycles persecond,

which corresponds to -a wave length, measured at ordinary roomtemperature, of approximately 22.3 inches. In this case 'a mean spacingbetween the sound transmitting and receiving openings of theloudspeakerand microphone, respectively, of approximately 5.6 inches maybe used to obtain the maximum of acoustical attenuation between the twoelements. Another factor which in part determines the acousticalcoupling between the two translating elements 40 l' and 402, forexample, is the size of the diaphragm used in the loudspeaker 102. Ingeneral, it may be stated that the smaller the loudspeaker diaphragm,the less the acoustical coupling between the loudspeaker and itsassociated microphone. In other words, a louddiameter is capable ofproducing a greater output per unit of diaphragm area without producinginstability of the station network. More specifically, it has been foundthat if the efiective diameter of the loudspeaker cone is made more thanone-half the minimum wave length of the operating frequency range, theacoustical coupling between the loudspeaker and the microphone elementsbecomes dimcult to control. In the system under consideration, atransmission band ranging from about 350 cycles per second to 5approximately 3300 cycles persecond is utilizedfor voice currenttransmission. Accordingly, a

' loudspeaker is provided which is equipped with a diaphragm having aneffective diameter of less than 6 centimeters.

assemblies are of the electro-dynamic type and. preferably, each thereofis of the form illustrated in Figs. 5, 6, 7, 8 and 9 of the drawings.These figures illustrate a working embodiment of the assembly, and inFig. 5 all parts are drawn to full size, scaled dimensions. In brief,the assembly there shown comprises a self-contained magnetic circuitwhich includes a permanently magnetized annular ring 500 formed ofAlnico, a bottom plate tuba center pole piece 502 and a top plate 603.r, The parts 50!, 502 and 503 are preferably formed of AlleghenyElectric metal or other v highly permeable magnetic material. The centerpole piece 502 is provided with a lower portion 05 502a of reduceddiameter, which extends within a. centrally disposed opening 'providedin the lower. plate Bill. -A'sweat connection between the side walls ofthis opening and the sides of the extended portion 502a is used rigidlyto secure a the center pole piece ililto the bottom plate 50!.

a At its upper end the center pole piece 502 is provided with a portion50% of reduced diameter, which extends within a centrally disposedopening 503a formed in the top plate 503. As best 7 shown in Fig. 50fthe drawings, the diameter speaker unit having a small effectivediaphragm The individual microphone and loudspeaker of the opening 503ais slightly larger than the diameter of the upper portion 50% of thepole piece 502. The upper end of the center pole piece is concentricallydisposed within the opening 5031a, whereby an annular air gap is formedbetween the adjacent side wallsof the opening and the center pole piece.The magnetic circuit assembly is completed by means of three clampingscrews 5041 which extend through openings provided in the bottom plate58! and are threaded into tapped holes drilled in the top plate 503.These screws serve rigidly to clamp the annular permanent magnet 500between the top and bottom plates 583 and 5M.

The moving system of the translating device comprises a cone-shapeddiaphragm 505 which is formed of pressed fiber and is carried by asupporting structure which comprises a bottom ring 506, six spacingcollars 501, a pair of clamping rings 508 and 509, two paper washers 5mand 5i i, and six assembly screws 5l2. Thecharacter of the diaphragmdepends upon whether the device is to be used as a transmitting orreceiving element. If the device is to be used as a transmitter, thediaphragm is of one-piece construction and is provided with an outerfiat annular ring portion 565a which extends between and is cemented. tothe paper washers 5| and H and is uniformly clamped around its peripherybetween these washers by the two clamping rings 5% and 509. If thedevice is to be used as a receiving element, the diaphragm constructionillustrated in Fig. 8 of the drawings is used. In this construction onlythe bulbous portion of the diaphragm 505 is constructed of molded or.pressed fiber, the periphery of this element being secured to an annularring 5051; formed of treated silk which is disposed'between and cementedto the paper washers 5H] and 5! l. The purpose of this arrangement is topermit substantially free vibration of the diaphragm element 505,particularly at the low frequencies of the operating frequency range,whereby the response of the device at these frequencies is enhanced.With the onepiece diaphragm arrangement, on the other hand, thediaphragm is considerably stiffer and, ac-

cordingly, the response thereof, particularly atlow frequencies, issubstantially less than when the Fig. 8 arrangement is used. By virtueof this increased stillness, low frequency noise components' of soundare prevented from producing any substantial response of the device.

A moving coil 5| 3 is cemented to the lower bulbous portion of thediaphragm 505 and is concentrically disposed within the air gap definedby the upper portion 50212 of the pole'piece 592 and the opening 503a inthe top plate 503. The terminal ends of this coil may be electricallyconnected to lead-in wires in any-desired manner although, preferably,the connections are made atv anchor posts'which extend through and aresecured to the lower bottom portion of the diaphragm 585. In order topreserve the correct lateral and axial spacing of the moving c'oil M3 inthe. air gap of the magnetic circuit,-and to provide additional supportfor the diaphragm. 505, a string supporting arrangement is provided.

This arrangement comprises three connected strings Etta, 5 Nb and 5Hewhich extend through equi-angularly spaced openings cut through the.

clamping the same to the lugs 5l5a, 5 l5b and 5i5c which extend radiallyinward and are turned upward from the supporting ring 506. Smallclamping plates 5|6a, 5i6b and 5l6c disposed beneath shortened spacingcollars 501 are used to clamp the free ends of the strings 5l4a, Sllband'5l4c against the lugs 5l5a, 5i5b and 5|5c. Preferably the threestrings 5l4a, 5ND and 5l4c are formed of silk fishline or the like, andit will 'be understood that by appropriately tensionlng the threestrings the required support for the moving coil 5 i 3 may be obtained.

In order to seal the annular cavity, formed between the spaced apartannular permanent magnet 500 and the pole piece 592, from the acousticalcavity 5H adjacent the lower side of the diaphragm 505, thereby toenhance the damping of the diaphragm and thus produce a more uniformresponse thereof over the entire operating frequency range, a sealingring 5 I 8 is provided which snugly encloses the upper end portion 502?:of the center pole piece 502, and is clamped to the underside of the topplate 583 by means of as-' sembly screws 5 i 9. This ring is preferablyformed of rubber, molded Bakelite or other insulating material havingnon-magnetic properties.

In the assembly of the translating device the magnetic circuit structureis built by first securing'the sealing ring bit to the underside of thetop plate 593 and then setting the assembly screws 505 to clamp theannular permanent magnet 50. between the top and bottom plates 503 and5M. After the magnetic circuit structure is thus assembled the portionof the sealing ring 5! facing outward through the air gap of thestructure may be painted with shellac or other sealing compound, toprovide an acoustical seal which blocks oil'the cavity oi'the magneticcircuit structure from the cavitylii'l adjacent the underside of thediaphragm 505. The diaphragm assembly is prepared by cementing the twopaper washers III and 5| l to the top and bottom surfaces of the flatannular portion 505a of the diaphragm 505, assuming that a transmittingelement is being constructed, and by cementing the moving coil III tothe lower central portion of the diaphragm member. Ifthe device is to beused as a loudspeaking receiver element, the'diaphragm structureillustrated in Fig. 8 of the drawings is constructed by first cementingthe paper washers Ill] and III to the opposite peripheral surfaces ofthe silk ring 505b, after which the inner upper edge surfaces of thesilk ring are cemented to the underside of the diaphragm 505. Followingthe construction of the diaphragm structure, the three strings Ella,5-l4b and 5H0 are drawn through the angularly spaced openings providedin the base of the diaphragm 505. After this operation is performed, thesupporting ring 506, the clampins rings 508 and 509 and the assembleddiaphragm structure may be stacked on the top plate 503, and theassembly screws 5|! may successively be inserted through the registeringopenings provided in the stacked clamping rings and diaphragmstructura'the spacing collars "L gand the supporting ring 506. Initiallythe screws which extend through the clamping elements III are nottightened, but the alternate assembly screws which extend only throughalternate spacing collars 501 are tightened sumciently to provide asemi-rigid assembly. Following this opera.- tion the ends or the stringsElla, Bill: and lllc may be threaded between the clamping elements 5i6a,Mao and 5150 and their associated lugs 855a, blob and biiia and properlytensioned to provide the required lateral and axial support for deviceis completed.

As pointed out above, each transmitting and receiving element, asconstructed. in the manner just described, is preferably embedded in ablock of acoustical insulating material to prevent the transmission ofsound waves and mechanical vibrations to themoving system of theelement. In the arrangement illustrated in Fig. of the drawings, thetranslating device is disposed in an opening formed in a block ofacoustical insulating material 520. This block of insulating material,which may be of the desired configuration, and morespecificallymay be ofany configuration illustrated in Figs. 4a, 4b, 4c, 4d and 4e of thedrawings, is preferably preformed to the desired shape in accordancewith the method de scribed and claimed in co-pending application SerialNo. 467,695, filed Dec. 3, 1942, Giannini. In brief, the method theredisclosed comprises mixing a loose insulating material of the rock wooltype with a cement binder, and molding the mixture into a solid block ofthe desired shape before drying of the binder starts. The molded blockobtained after the binder is fully set, is somewhat porous and willstill transmit air through some of its surfaces.

Accordingly the outer surfaces of the molded block are painted with oneor more coats of lacquer to minimize sound transmission therethrough. Ithas been found that the number of coats or the thickness of thelayer'oflacquer on the outside surfaces of r the molded block determine,to some extent, the acoustical permeability of the structure. Hence, byappropriately controlling the depth of the lacquer layer, the acousticalcharacteristics of the ings. More specifically, the Fig. 9 arrangementcomprises a block'of acoustical insulating material 52! which isprovided with a number of sound transmitting passages 522 molded or cuttherethrough along the axis of sound-transmis- "sion to and from thediaphragm of the associated transmitting or receiving element. Thesepassages are ofthe correct dimensions, 1. e., diam-j eters and lengths,to provide'for maximum sound energy transfer between the atmosphere andthe cavity immediately adjacent the outer surface of the diaphragm. Theblock of acoustical insulating material 52l maybe secured to the blockof insulating material 62!! to clamp the'aluminum sheet 524 and the silkscreen 523 'to the top surface of the block 520 by means of assemblyscrews passed through registering openings in the two insulating blocks.

From the foregoing explanation it will be understood that a transmittingand a receiving element, each constructed in the improved mannerillustrated in Figs. 5 to 8, inclusive, of the drawings, may be mountedin one and the same block of acoustical insulating material. This blockmay be preformed to provide the desired mean spacing between the soundtransmitting and sound receiving openings adjacent the diaphragms of theloudspeaker and microphone elements, respectively. It will also beunderstood that in the unit as thus formed the conductive couplingbetween the two elements is reduced to a minimum by virtue of thevibration absorbing characteristics of the acoustical block. Moreover,the small effective diameter of the loudspeaker element, as shown inFig. 5 ofjthedrawings, contributes materially to the acousticalattenuation between the transmitting and receiving elements. Thisattenuation may also be enhanced by utilizing pedance of air, over whichis placed a thin sheet of aluminum 524. having a number of holes ofappropriate size cut therethrough for the purpose of transmitting soundto or from the diaphragm of the enclosed element. A clamping ring andassembly screws, which are passed through registering openings in theclamping ring, the aluminum plate .524 and the molded block ofinsulating material, may be used to 'hold the aluminum plate 524 and thesilk screen 523, in assembled relationship on the block of insulatingmaterial.

' If directional characteristics are to be imparted to the device, anadditional block of molded acoustical insulating material, constructedin accordance with the method briefly outlined above, and having soundtransmitting passages of apprcpriate lengths and dimensions preformedtherein, may be provided. A simple embodiment of an acousticalinsulating block provided with such passages is illustrated in Fig. 9 ofthe drawings as being adapted for use in conjunction with the unitillustrated in Fig. 5 of the draw-v .the sound directing arrangementsillustrated in Figs. 4b, 4c, 4d and 4e of the drawings in the amplifiersat each of the two connected stations I0 and H, as well as the channelamplifiers in the central repeater l2, may be operated with normal gainsettings which are substantially greater than the permissible gainsettings when conventional microphone and loudspeaking ar.- rangementsare used.

In considering the operation of the system, it

may be assumed that, the signal amplifiers and the control network, asprovided at each of the two stations in and H, are active, and that thecentral repeater I2 is conditioned for operation."

In this regard it will be noted that to condition the circuit equipmentprovided atthewest stae tion H for operation, the switch 838 ls'manuallyoperated to its closed-circuit position wherein alternating current isdelivered to the parallelconnected windings 355 and 331 of the powertransformers 35 4 and 336. With these transformers energized, lowvoltage'alternating current is deliveredto the cathodes of each" of thetubes342, 322, 326, 830 and 308 over obvious circuits. With therectifier tube 342 in operation, current alternately traverses the twoanodes of the tube 342 during alternate half-cycles of ode acsc.

the voltage developed across the high voltage secondary winding 338, andis passed through each of the two resistors 3 53 and 3 39 in the samedirection. The alternating components of the voltage thus developedacross the resistor 349 are minimized through operation of the filternetwork comprising the choke coil 346 and the shunt-connected condenserst ll and 368. The

direct voltage developed across the section 343a of the resistor 343 isnegatively applied to the control electrode Nth of the transmittingamplifier tube '306 to determine the operating point on thecharacteristic of this tube. Similarly, the voltage drop across theresistor section 3532) is negatively applied to the control electrode3301) of the tube 330 to determine the normal space current flow throughthis tube.

Initially, the voltage dividing resistor 385 is so adjusted that themaximum signal output from the amplifier 3M consistent with stability ofthe station circuit is obtained when heating current of rated'valuetraverses the cathode 3850 of the tube 39%. Similarly, the voltagedividing resistor dill is initially adjusted so that the maximum signaloutput from the amplifier till consistent with stability of the stationcircuit is obtained when the amplifier 3M is operating with normalsignal current gain therethroughl In this regard it will be apparentthat the stability of the system is determined by the combined signalcurrent gains through the two amplifiers 30B and 8M and, hence, the gainsetting of each amplifier is dependent upon the setting of the otheramplifier. Assuming that the on-ofi" switch 386 occupies the positionillustrated in the drawings, the signal current output from thereceiving amplifier $68 is in part determined by the setting of theadjustable voltage dividing resistor 39! and in part by'the resistanceof' the shunt connected space current path of the control tube 330. Theresistance of the space current path of the tube 33!} depends, in turn,upon the negative bias voltage impressed across the input electrodes83012 and 8300 thereof. Normally this voltage is equal to the sum of thevoltage drop across the'resistor section 3431) and the voltage dropacross the cathode biasing resistor 352. The total magnitude of thisbias voltage is normally of the order of 23 volts.- With the two tubes308 and 330 thus conditioned for operation, the current traversing theparallel-connected space current paths there- 308. receiving terminalsof the hybrid system 203 is transmitted through the coupling transformerassume the setting of the adjustable cathode biasin resistors 235a and235b. Each of these resistors is initially adjusted so that the spacecurrent flow through the associated tube develops 9. voltage ofapproximately 12 volts across the associated resistor 236a. or 23617;The voltageacross the resistor 236a is negatively applied to the controlelectrode 20'") of the west-east amplifier tube 201. Similarly thevoltage developed across the resistor 23Gb is negatively applied to thecontrol electrode 2 l 2b of the east-west amplifier tube 2 I 2.

Assuming that the microphone-loudspeaker unit illustrated in Fig. 4a ofthe drawings is utilized in conjunction with the circuit equipmentillustrated in Fig. 3 of the drawings and that a user of the stationequipment speaks into the microphone 40 l the signal voltage developedin the moving coil of this microphone is transmitted through thecoupling transformer 304 and the portion of this voltage appearingacross the lower portion of the voltage dividing resistor 365 isimpressed upon the input electrodes 30% and 3960 of the transmittingamplifier tube 308. The signal currents as amplified by the tube 306 aretransmitted through the coupled windings of the hybrid system 302 andover the conductors of the line H to the windings of the hybrid system203. By virtue of the arrangement of the coupled windings of the hybridsystem 302 and the action of the balancing network.303, only a smallportion of the signal current: energy is transmitted through the hybridsystem to the input electrodes of the receiving amplifier tube Thesignal voltage appearing across the 206. and is impressed across theinput electrodes 20'": and 2010 of the west-east amplifier tube 201 inparallel with the primary winding of the coupling transformer 22L Thesignal currents as amplified by the tube 201 are transmitted through thecoupling transformer 203 and the coupled windings of the hybrid system202 to the line l3. From this point the signal currents are transmittedover theline. l3 and through the hybrid system I02 to the input circuitof the signal current amplifier I05. These currents as amplified by theamplifier I05 are transmitted to the moving coil of the loudspeaker [0|for reproduction. The signal voltage as impressed between the inputelectrodes 22312 and 2230 of the amplifier of divides between theresistor 852 and the cathode 3060 of the transmitting amplifier tube300, The value of the resistor 352 may be'so chosen that rated currentnormally traverses the oathtube 223 through the coupling transformer)and the voltage divider 222, is amplified through With the gain settingsof the two amplifiers $00 and $0! determined inthe above-describedmanner, the circuit constants of the station cir cuit are so chosen thatthe signal current gains- .through the two amplifiers 309 and 3M aresuboperation, but inactive, no signal currents are transmitted througheither of the two amplifier tubes 223 and 22? to either of the tworectifying sections of the duplex diode tube 23L and hence no biasvoltage is developed across the resistor 286. Accordingly the spacecurrent flow through the tubes 232 and 238 and their respectiveassoelated resistors 23% and 28th is determined by by way of therisistor 230, the anode am, the

1 space current path between this anodeland the cathode 23lc and theresistor 234 to the lower terminal of the indicated transformer winding.

. This pulsating current is smoothed through the action of the condenser233 which shunts the resistor 234. The voltage thus developed across theresistor section 234a is negatively applied to trade 20'": of the activewest-east signal amplifier transmitted through the coupling transformer2|! and thehybrid system 203 to the line I4.

' From this point they are transmitted over the 1y decreased. Thus itwmihe noted that the voltage drop across the resistor section 2341) ispositively applied to the control electrode of thetube 233, whereby thespace current flow through this tubeis increased. As a result thevoltage drop across the biasing resistor 23Gb is increased.

Since this voltage is'negatively applied to the control'electrode 2I2boi the tube 2l2, it will 'be understood that a corresponding decreasein'the signal current gain through the amplifier tube 2l2 occurs.

From the above explanation it will be understood that the increase insignal current gain through the active west-east transmission channel200 is accompanied by a1 corresponding decrease in the gain through theinactive east-west transmission channel 20! of the central repeaterv l2.- Thus the stability of the repeater network is preserved. In thisregard it will be noted that the network includes a substantially closedsignal current circuit which comprises the two channels 200 and 20! andthe coupling paths through the two hybrid systems 202 and 203, and thatif the two amplifier tubes 201 and M2 are both operated at the high gainsettings required during signal current transmission, the total gainaround the closed circuit may substantially exceed the electrical lossesof the circuit. When this condition prevails the circuit network is.highly unstable and an oscillatory condition maybe set up therein evenwhen signal currents of relatively small amplitudes are transmittedtherethrough. Due to the action of the control network.220, however, thetotal gain around the closed circuit is maintained below thepredetermined value at'wh'ich.

the circuit is stable by concurrently increasing the gain of the channelin use and decreasing the gain of the inactive channel. Accordingly, thestability of the repeater is preserved even though signal currents ofsubstantial magnitude are transmitted through the repeater. Each timesignal current transmission over the line I4 is terminated to arrest thesignal input to the control network 220, the bias voltages across thetwo resistor sections 234a and 23% are reduced to zero, whereby thenormal bias voltages across the two biasing resistors 230a and 23612 arerestored. The signal current gain through the amplifier tube 201 is thusdecreased to normal and the signal current gain through the amplifiertube 2| 2 is increased to itsnormal value. The rate at which the biasvoltages across the resistors 236a and 23Gb are restored to normal isprimarily determined by thetime constant of the network comprisingthe-resistor 234 and the condenser 236. The resistance and capacitancevalues of this network are proportioned to provide the necessaryhangover period at each signal current break-off point, in order toprevent syllable clipping which would otherwise occur.

When sound waves are transmitted to the microphone I00 provided at theeast station l0, corresponding signal currents are developed in theoutput circuit of the signal current amplifier I04 and are transmittedthrough the hybrid system I02, over the line l3 and through. the hybridsystem 202 to the parallel-connected input circuits'of the channelamplifier tube 2l2 and the control amplifier tube 221. These signalcurrents as amplified by the channel amplifier 20! are line H andthrough the hybrid system 302 to the input circuit of the receivingamplifier 30! provided at the West station ll. After being amplified bythe receiving amplifier tube 308, the

signal currents are transmitted through the.

' coupling transformer 309 and over the cord conductors 43! to themoving coil of the loudspeaker 402 for reproduction.

The signal voltage as impressed between the input electrodes of theamplifier tube 221 through the coupling transformer 225 and the voltagedivider226 is amplified'by this tube and impressed through the couplingtransformer 220 I across the anode 23ld and the cathode 23Ie of theduplex diode 23L More specifically, the'voltage appearing across thesecondary winding of the transformer 228 causes a pulsating directcurrent to traverse the. resistor-234 in a direction which may be tracedas extending from the upper terminal of this winding by way of theresistor 229, the anode 23ld, the space current path between this anodeand the cathode 231e, and the resistor 234 to the lower terminal of theindicated t'ransformer'winding. This pulsating current is smoothedthrough the action of-the condenser 236, so that a substantially steadydirect voltage is developed across the resistor 234. The portion of thisvoltage appearing across the resistor section 234a is positively appliedto the control electrode of the tube 232 to increase the space currentflow through this tube and thus increase the voltage drop across thebiasing resistor 236a. When this voltage drop is increased the negativebias on the control electrode 20111 of the inactive west-east signalamplifier tube 201 a is increased to decrease the amplification factorof this tube. At the same time that the gain throughthe inactivewest-east channel 200 of the repeater I2 is thus decreased, the gain ofthe active east-west channel 20l is correspondingly increased. Thus itwill be noted that the voltage drop across the resistor section 2341) isnegatively applied to the control electrode'of the tube 233 so that thespace current flow through this tube is decreased. As a result, thevoltage drop across the biasing resistor 23Gb is decreased. Since thisvoltage is negatively applied to the control electrode 2l2d of the tube2 l2, it will be understood that a decrease in the magnitude of thisvoltage produces a corresponding increase in the signal current gainthrough the amplifier tube 2l2.

Thus it will be seen that when signal currents are transmitted in theeast-west direction from the station I 0 to the station I I, the signalcurrent gain through the active channel 20l is increased the requiredamount for satisfactory reproduc- 'tion of the signal currents by theloudspeaker 402 in use, and this signal current gain is accompanied by acorreponding decrease in the gain of the inactive west-east channel 200,whereby the stability of the central repea er I2 is preserved.

It will also be understood from the above explanation that each timesignal current transmission over the line l3 from the east station I0 isterminated to arrest the signal input to the con-' trol network 220, thebias voltages across the two resistor sections 23% and 2341) are reducedto zero after a short time interval which is determined by the timeconstant of the shunt circuit including the'resistor 234 and thecondenser 230.

- Accordingly, the bias voltages across the two biasing resistors 236aand-236b are again equalized at their respective normal values, wherebythe signal current gain through the amplifier tube;

201 is increased to normal and the signal current gain through theamplifier tube 2i2 is decreased to its normal value. I

Referring now more particularly to the method of controlling the network3i5 provided 'at the west station Ii, it will be notedthat a portion ofthe signal voltage appearing across the output circuit of the receivingamplifier tube 308 is imthat the ratio of the signal voltages across thetwo resistors 3iil and 32i is determined by the reactance of thecondenser 320. At relatively low signal voltage frequencies thereactance of this condenser is relatively high so that a relativelylarge proportion of the available voltage drop across the twoseries-connected resistors 3i8 and 32i appears across the resistor 32i.On the other hand, as the signal current frequency increases, acorresponding decrease occurs in the reactance of the condenser 320, sothat an increasing percentage of the available signal voltage appearsacross the resistor 3i8. Thus by properly proportioning the constants ofthe network 3i8, this network may be operated to favor signal currentsof the frequencies within the band indicated. The reason for providing adiscriminating network 3i6 which favors signal current frequencieswithin the low end of the operating range, is to .make the controlnetwork 3i5 primarily responsive to the band of frequencies at which themajor portion of the voice current energy is produced. Thus thepredominant portion of the frequencies used in ordinary speech lieswithin the band ranging from 300 to approximately 1200 cycles persecond. The signal voltage as developed across the resistor 32l isamplified by the,tube 322, and the amplified signal currents causecorresponding induced voltages in the secondary winding of the. couplingtransformer 32, which voltages are applied across the anode and cathodeof the rectifying tube 326. This tube is biased by means of the battery32! to a point on its operating characteristic such that no current willtraverse the space current path thereof until the signal voltageappearing across the secondary winding of the transformer 320 exceeds apredetermined value. Thus, so long as the signal voltage across theoutput circuit of the receiving amplifier tube 308 does not exceed apredetermined value, the biasing battery 328 prevents the controlnetwork 3i! from changing the gain of either of the two amplifiers 30ior 302. When, however, the sig-- nal voltage appearing across the outputcircuit across the resistor 328 is additive with respect to the biasvoltage across the section 33% of the resistor 343. Accordingly the netbias yoltag across the inputelectrodes 330D and 338c of the control tube330 is increased to increase the resistance of the space current path ofthis tube. Incident to this increase in space current resistance of thetube 330, the shunting effect of the tube on the output circuit ofthereceiving amplifier tube 308 is correspondingly decreased to producean increase in the signal current gain through the amplifier 30i. Thisdecrease in the shunting effect of the tube 330 is supplemented by anincrease in the amplification factor of thespondingly decreased, so thata larger portion of the available voltage across the resistor 349 isapplied to the anode 308a of the tube 308. The

- increase in gain through the amplifier 30l is accompanied by acorresponding or even greater decrease in the gain of the transmittingamplifier 300. Thus when the resistance of the space current paththrough the tube 330 is increased, the direct current traversing thipath and the shunt-connected resistor 352 and cathode 306c iscorrespondingly reduced. When thecurrent traversing the cathode 3080 isreduced, the electron emission of this tube is decreased to lower theamplification factor of this tube. From the above explanation it will beunderstood that the signal current amplitude at which the controlcircuit 3i5 starts inversely to change the gains of the two amplifiers300 and 3!, may be adjusted, through suitable adjustment of the voltagedivider 3l8, to vary the proportion of the available signal voltagewhich is impressed between the input electrodes of the tube 322. Thisadjustment is determined to a large extent by the normal gain settingsof the two amplifiers 300 and 301. Hence, after the required signalcurrent gain through the two tubes is once established, the voltagedivider 3i8 may be adjusted to provide the minimum signal voltage inputto the amplifier tube 322 consistent with the required control of thestation circuit.

It will also be understood from the above explanation that after thecontrol circuit 3i5 starts to operate, the extent to which the signalcurrent gains of the two amplifiers 300 and 30i are inversely changeddepends, within limits, upon the magnitude of the signal currentsincoming to the west station Ii over the line It. Preferably, thecircuit constants of the network 3i5 are so adjusted that the amount ofthe decrease gain of the signal amplifier 300 which occurs in responseto any given change in the input signal intensity is just sufilcient tomaintain the stability of the station circuit. As indicated by the aboveexplanation, the gain. control action realized through operation of thenetwork 3i! is not effective until the signal input to the loudspeaker402 exceeds a predetermined value. By virtue ofthis arrangement, thecontrol network "3i! is rendered substantially non-responsive to noisecurrents resulting from'background noises which may be acousticallyimposed on the systemor to noise currents resulting from electrical,

transients imposed on the system, which currents are usually of a loworder .of magnitude. Since such noise currents are usually of lowfrecuency, the coupling condenser 3" may be chosen of such value as toprevent any substan- .tial portion of the resulting voltage across theaccuse Thus, the response of the network 3l5 is limited to speech orother actual signal current transmission over the two connected lines ofthe sys-: tem in an east-west direction. Further, the constants of thestation circuit are so chosen that thevariation in gain of the amplifier300,. cffected through operation of the control network 3l5, are limitedto a definite range which will include all cases of normal signalcurrent intensities. I

In the event it is desired to render the control .network 3|5 inactive,the on-off switch 36! is operated to its 011 position whereintheparallel connection between the space current paths of the two tubes308 and 3301s broken and anode potential is supplied to the control tube330 directly from the high voltage terminal of the load and an. has beenillustrated in the drawings, it

will be understood that a second control network of like arrangement andresponsive :to*signal currents transmitted from the microphone l to theamplifier 300,-may be provided ifnecessary. In such case, the input andoutput cir-. cults of the second control network will be bridged acrossthe output circuit of the amplifier tube 306 and the space currenttraversing the control tube of the second network will determine thecathode heating current of the receiving amplifier tube. With such anarrangement the second control circuit will function to increase thegain I or the transmittingamplifier tube 306 and to resistor 303included in the power pack 335. With the switch 36l in its off position,it will be apparcut that amplified signalvoltages appearing across theoutput circuit of the tube 306 are not' While it will be understood thatthe specifications or the circuit provided at the station H may varyaccording to the designof a particular installation, the followingspecification of circuit constants for the station circuit of Fig. 3 isincluded by way of example as being satisfactory.

Tube 306 Commercialtype 4'7 Tube 306---: Commercial type 1A5G Tube 322Commercial type 26 Tube 326 Commercial type 26 Tube 330... Commercialtype 10 Tube 302 Commercial type Condenser 363 0.25 microfarad Condenser365 50.0 microfarads Condenser 3H 0.001 microfarad Condenser 320 0.002microfarad Condenser 323. 0.05 microfarad Condenser321 8.0 microfaradsCondenser 331 0.25 microfarad Condenser 323-; 0.01 mlcrofarad Condenser305 8.0 microtarads Condenser 300-1 8.0 microfarads Condenser 308 16.0microi'arads Condenser 301 8.0 microfarads Condenser 353 5.0 microiaradsCondenser 360 -4 5.0 microfarads Resistor 305 l 250,000 ohms Resistor360 i 500,000 'ohms Resistor 366 500,000 ohms Resistor-30L 250,000 ohmsResistor 362 1250 ohms Resistor 3l8 250,000 ohms Resistor 3J9 100,000ohms- Resistor 32l 500,000 ohms Resistor 326 500,000 ohms Resistor 303250 ohms Resistor 303 25,000 ohms Resistor 350 15,000 ohms Resistor 351420 ohms Resistor 352 ohms Resisto 350 1000 ohms Battery 325 4.5 voltsAlthough only one control network3l5, which responds to signal currentsincoming to the station H to control the gains of the amplifiers 300decrease the gain of the receiving amplifier tube when sound wavesexceeding a predetermined value are transmitted to the microphone 00I.

The control circuit shown in Fig. 2 of the drawings of the presentapplication is claimed in the copending application of Giannini andEisenberg.

Serial No. 472,722, filed January 18, 1943, and the control circuitshown in Fig. 3 of the drawings of the present application is claimed inthe copending application of Giannini and Eisenberg. Serial No. 419,164,filed November 14, 1941.

. While one embodiment of the invention has k been described, it will beunderstood that various modifications may be made therein withoutdeparting from the true spirit and scope of the invention.

What is claimed is: 1. In a loudspeaking intercommunicating systemcomprising a station network having acoustically coupled transmitter andreceiver elements 5 which are electrically coupled to a two-waytransmission line over transmitting and receiving channels respectively,and wherein a signal amplifier is'provided which'tends to impartinstability to said network, the method of operation which comprisesdirectionally transmitting sound waves produced by said receiver elementaway from the response zone of ,said transmitter along a substantiallyvertical path; altering the gain of said amplifier in the correct senseand only by the required amount to maintain the stability of said.network'when signal currents exceeding a predetermined value traversesaid receiving channel, and maintaining the gain of said amplifier atthe maximum level consistent with stability of said network when signalcurrents not exceed-* i118 said predetermined value traverse saidreceiving channel. Y r 2. In a loudspeaking intercommunicating system. anetwork comprising a transmitting element disposed to respond to soundwaves trans- I mitted theretoalong a substantially horizontal plane, areceiving element disposed to transmit sound waves produced thereby in avertical direction, variable gain transmitting and receiving channelsrespectively coupled to said transmitting and receiving elements, meansfor automatically decreasingthe gain of. one of said channels only whenthe signal input to the other of said channels exceeds a predeterminedvalue, and means for limiting the. decrease inthe gain of said onechannel to an amount just suificient to maintain the stabllityof saidnetwork.

3. In a loudspeaking intercommunicating system, a network comprising areceiving element for directionally transmitting sound waves producedthereby along two spaced apart and diverging paths, a transmittingelement disposed substantially equidistant between said paths, wherebysound energy transfer from said receiving element to said transmittingelement is mini- 12 mized, variable gain transmitting and receiving 3channels respectively coupled to said transmitting and receivingelements, means for automatically I decreasing the gain of one of saidchannels only when the signal input to the other of said channelsexceeds a predetermined value, and means for-limiting the decrease inthe gain of said amplifier to an amount just suflicient to maintain thestability of said network. v I

4. In a loudspeaking intercommunicating system, a network comprisingtransmitter and re- ,ceiver elements having a combined frequency re-.sponse characteristic which peaks at a predetermined frequency withinthe voicefrequency range and having their sound receiving and discharge.

openings spaced apart a distance equal to substantially an odd number oftimes one quarter of the wave length which corresponds to said frequency, variable gain transmitting and receiving channels respectivelycoupled to said transmitting and receiving elements, means forautomatically decreasing the gain of one of said channels only when thesignal input to the other of said channels exceeds a predeterminedvalue, and means for limiting the decrease in the gain of said one saidlower portion of the voice frequency range for automatically decreasingthe gain of one of said channels only when the signal input to the otherof said channels exceeds a predetermined value.

6. In a loudspeaking intercommunicating system, a network comprisingtransmitter and receiver elements having a combined frequency responsecharacteristic which peaks at a predetermined frequency within the lowerportion of the voice frequency range, variable gain transmitting andreceiving channels. respectively-coupled to said transmitting andreceiving elements, means comprising a frequency discriminating circuitwhich favors the frequencies within said ,lower portion of the voicefrequency range for automatically decreasing the gain of one of saidchannels only when thesignal input to the other of said channels exceedsa predetermined value,

.and means for limiting the decrease in the gain of said one channel toan amount just sufficient to maintain the stability of said network.

7. In a loudspeaking intercommunicating system, a network comprisingtransmitter and receiver elements having a combined frequency responsecharacteristic which peaks at a predetermined frequency within the lowerportion of the voice frequency range and having their sound receivingand discharge openings spaced apart a distance substantially equal to anodd number of times one-quarter of the wave length which corresponds tosaid frequency, variable gain transmitting and receiving channelsrespectively coupled to said transmitting and receiving elements, andmeans comprising a frequency discriminating circuit which favors thefrequencies within said lower portion of the voice frequency range forautomatically decreasing the gain of one of said channels only when thesignal input to the other of said channels exceeds a predeterminedvalue.

8. In a loudspeaking intercommunicating sysdetermined frequency withinthe lower'portion of the voice frequency range and having their Y soundreceiving and discharge openings spaced apart a distance substantiallyequal to an odd number of times one-quarter of the wave length whichcorresponds to said frequency, variable gain transmitting and receivingchannels .respectively coupled to said transmitting and receivingelements, means comprising a frequency discriminating circuit whichfavors the frequencies within said lower portion of the voice frequencyrange for automatically decreasing the gain of one of said channels onlywhen the signal input to the other of said channels exceeds apredetermined value, and means for limiting the decrease in the gain ofsaid one channel to an amount just sufficient to maintain tern, anetwork comprising a transmitter elev ment, a loudspe ment, aloudspeaking receiver element including a vibrating diaphragm having aneffective diameter less than one-half the wave length which correspondsto the highest frequency of the operating frequency range of the system,variable gain transmitting and receiving channels respectively coupledto said transmitting and receiving elements, and means for automaticallydecreasing the gain of one of said channels only when the signal inputtothe other of said channels exceeds a predetermined value.

10. Ina loudspeaking intercommunicating system, a network comprising atransmitter element, a loudspeaking receiver element including avibratory diaphragm having an effective diameter less than one-half thewavelength which corresponds to the highest frequency of the operatingfrequency range of the system, said elements having a combined'frequencyresponse characteristic which peaks at a predetermined frequency withinsaid'range and being disposed so that their sound receiving anddischarge openings are spaced apart by a-distance which is substantiallyequal to an odd multiple of onequarter of the wave length whichcorresponds to said predetermined frequency, variable gain transmittingand-receiving channels respectively. coupled to said transmitting andreceiving elements, and means for automatically decreasing the gain ofone of said channels only when the signal input to the other of saidchannels exceeds a predetermined value.

11. In a loudspeaking intercommunicating system, a network comprising atransmitter eleaking receiver element including a vibrating diaphragmhaving an effective diameter less than one-half the wave length whichcorresponds to the highest frequency of the operating frequency range ofthe system, said elementshaving a combined frequency responsecharacteristic which peaks at a predetermined frequency within the lowerportion of said range, variable gain transmitting and receiving channelsrespectively coupled to said transmitting and receiving elements, andmeans comprising a frequency discriminating circuit which favors thefrequencies within said lower porton of said range for automaticallydecreasing the gain of one of said channels only when the signal inputto the other of said channels exceeds a predetermined value.

12. In a loudspeaking intercommunicating system, transmitter andreceiver elements havin a combined frequency response characteristicsaid elements having their sound receiving and discharge openingsspacedapart by a distance substantially equal toan odd multiple of onequarterof the wave length which corresponds to said predeterminedfrequency.'varlable gain communication channels respectively coupled'tosaid elements, and means comprising a frequency discriminating networkwhich favors si nal frequencies within said lower portion of saidoperating frequency range for controlling 'the'gain of at least one ofsaid channels to prevent said system from oscillating due to undesiredcoupling between said channels.

14. In a loudspeaking intercommunicating system, a transmitter element,a loudspeakin'g receiving element which includes a diaphragm having aneffective diameter less than one-half of the wave length correspondingto the highest frequency of the operating frequency range of the system,said elements having a combined frequency response characteristic whichpeaks at a predetermined frequency within said range and having theirsound receiving and discharge openings spaced apart by a distancesubstantially equal to an odd multiple of one-quarter .of the wavelength which. corresponds to said predetermined frequency, andtransmitting and receiving channels respectively coupled to saidtransmitting and receiving elements.

15. In a loudspeaking intercommunicating systen, a transmitter element,a loudspeaking receiving' element which includes a diaphragm having aneffective diameter less than one-half of the wave length correspondingto the highest frequency of the operating frequency range of the system,said elements having a combinedfrequency response characteristic whichpeaks at a predetermined frequency within-the lower.

portion of said range and having their sound receiving and dischargeopenings spaced apart by a distance substantially equal to an oddmultiple of one-quarter of the wave length which corresponds to saidpredetermined frequency, variable gain communication channelsrespectively coupled to said elements, and means comprising a. frequencydiscriminating network which favors signal frequencies within said lowerportion of said operating frequency range for controlling the gain of atleast one of sale. channels to prevent said system from oscillatin dueto undesired coupling between said channels.

16. In a loudspeaking intercommunicating system, a transmitter elementand a lcudspeaking' receiver element which includes a diaphragm havingan elective diameter less than one-half the wave length corresponding tothe highest frequency of the operating frequency rangeof the system, andtransmitting and receiving channels respectively coupled to saidtransmitting and receiving elements.

1'1. In a loudspeaking inter-communicating sys.

tem, a transmitter element and a loudspeakin. receiver element whichincludes a diaphragm having an effective diameter less than. one-halfthe wave length corresponding to the highest frequency of the operatingfrequency range of the system, said elements having a combined frequencyresponse characteristic which peaks at a predetermined frequency withinthe lower portion of said operating frequency range, yariabla gaincommunication channels respectively coupled to said elements, and meanscomprising a frequency discriminating network which favors signalfrequencies within said lower portion of said operating frequency rangefor controlling the gain of at least one of said channels to preventsaidsystem from oscillating due to undesired coupling between said channels.

GABRIEL M. GIANNIN'L

